1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to the method for manufacturing the semiconductor device provided with a capacitor (information storage capacitance element), which is formed using a metal oxide film such as a tantalum oxide (Ta2O5) film as a capacitive insulating film.
The present application claims priority of Japanese Patent Application No. 2002-194006 filed on Jul. 2, 2002, which is hereby incorporated by reference.
2. Description of the Related Art
Large Scale Integrations (LSIs) known as a representative of semiconductor devices are roughly classified into memory products and logic products, the former of which, in particular, has been developed remarkably with advancing semiconductor device manufacturing technologies in recent years. Further, the memory products are classified into Dynamic Random Access Memories (DRAMs) and Static Random Access Memories (SRAMs), most of which are made up of Metal Oxide Semiconductor (MOS) transistors excellent in integration density. Further, the DRAMs, in particular, are greatly advantageous over the SRAMs because of improvement in integration density as described above and hence because of a reduction in cost, thereby finding wide applications in a variety of memory units in information devices or a like.
In a DRAM, each memory cell includes a memory cell selecting transistor made up of an MOS type switching transistor and a capacitor connected to the memory cell selecting transistor, to store information in accordance with whether charge is stored in the capacitor or not. Recently, however, an amount of information to be stored has increased with growth of an information-oriented society, to limit an area occupied by the capacitors to be formed on a semiconductor substrate. Therefore, it is necessary to produce increased capacitance of the capacitor of each memory cell. If the capacitor does not have capacitance large enough to store information, the DRAM readily malfunctions due to an influence of an external noise signal or a like, being subject to errors represented by a soft error.
Conventionally, as a capacitive insulating film of the capacitor of a DRAM, there has been used, for example, a silicon oxide (SiO2) film, a silicon nitride (SiN) film, a metal oxide film such as a tantalum oxide film or a like. Of these potential capacitive insulating films, especially, the tantalum oxide film, which is a metal oxide film, has a large permittivity as compared to the silicon oxide film, the silicon nitride film, or the like. Therefore, by using the tantalum oxide film as the capacitive insulating film, it is possible to from the capacitor having large capacitance. Such the tantalum oxide film is typically formed by using a Chemical Vapor Deposition (CVD) method, because it is easy to do so.
When forming the tantalum oxide film by the CVD method conventionally, as described above, an oxidizing gas such as an oxygen gas is used together with a material gas containing tantalum in order to fill oxygen vacancies liable to occur in the tantalum oxide film when it is being formed and also to remove organic residue. If oxygen vacancies are generated in the tantalum oxide film, a leakage current may flow through the formed capacitor, resulting in formation of the capacitive insulating film having a poor film quality.
For example, Japanese Patent Application Laid-open No. Hei 7-14986 and Japanese Patent Application Laid-open No. 2001-35842 discloses a semiconductor device manufacturing method for introducing the material gas and the oxygen gas into a reaction chamber simultaneously to thus form the tantalum oxide film by a CVD method. Oxygen contained in the oxygen gas fills the vacancies and also removes the organic residue, thereby working to improve the film quality of the tantalum oxide film.
As shown in FIG. 10, formation of the tantalum oxide film is started by: placing a semiconductor substrate in a reactor (reaction chamber) kept at, for example, about 430° C. (deposition temperature) and at about 10 Torr (Torricelli) (deposition pressure); and, at a time t10, introducing into the reactor a material gas and the oxygen (O2) gas at a same time on condition of respectively predetermined amounts of flow, wherein the material gas is obtained by spray and vaporizing, for example, tantalum pentaethoxide [Ta(OC2H5)5: hereinafter may be referred to as PET], which is one of tantalum compounds, with a nitrogen (N2) carrier gas. At a time t20 when a predetermined time has elapsed, introduction of the material gas and that of the oxygen gas are stopped simultaneously. In this case, the oxygen gas is used to fill oxygen vacancies liable to occur when the tantalum oxide film is being formed as described above and also to remove organic residue. Further, by a conventional semiconductor device manufacturing method using such a deposition sequence, the material gas is introduced in one continuous step, to form the capacitive insulating film having a finally required film thickness.
If the material gas and the oxygen gas are introduced into the reaction chamber simultaneously to form the tantalum oxide film as in the case of the conventional semiconductor device manufacturing method, a partial pressure of the material gas decreases. As a result, step coverage of the formed tantalum oxide film is deteriorated, so that film thickness of the capacitive insulating film formed on a surface of a lower electrode is liable to be non-uniform, thereby making it difficult to obtain stable capacitance of the capacitor when it is formed. In particular, when the lower electrode is a three-dimensional structure such as a cylinder or its surface shape is formed like a Hemispherical Silicon Grain (HSG) in order to increase the capacitance, the step coverage is deteriorated remarkably, to give rise to such a problem that a desired capacitance value cannot be obtained or upper and lower electrodes short-circuit with each other.
If the oxygen gas is introduced less to improve the step coverage, on the other hand, the oxygen works less correspondingly, to cause oxygen vacancies to occur in the tantalum oxide film and also organic residue to be accumulated, so that the film quality of the tantalum oxide film is deteriorated inevitably. Further, if a deposition rate is decreased by lowering a deposition temperature in order to improve the step coverage, not only the film quality is deteriorated but also much time is spent to form a film having a finally required film thickness.
A tantalum oxide deposition method for solving these problems by introducing the material gas in a non-oxidizing atmosphere to continuously form the tantalum oxide film until it has a finally required film thickness and then annealing the film in an oxidizing atmosphere to fill oxygen vacancies that have occurred in the tantalum oxide film is disclosed in, for example, Japanese Patent Application Laid-open No. 2000-340559.
However, by the tantalum oxide deposition method disclosed in Japanese Patent Application Laid-open No. 2000-340559, the tantalum oxide film is continuously formed until it has a finally required film thickness and annealed in the oxidizing atmosphere, thus giving rise to a problem that oxygen vacancies generated in the tantalum oxide film cannot completely be filled.
That is, by a semiconductor device manufacturing method by use of the tantalum oxide deposition method disclosed in Japanese Patent Application Laid-open No. 2000-340559, when forming the tantalum oxide film which makes up the capacitive insulating film of the capacitor, the oxygen gas is not introduced simultaneously with the material gas and so not influenced by the material gas, thereby improving step coverage. On the other hand, the material gas is continuously introduced at a constant flow rate to form the tantalum oxide film until it has the finally required film thickness and then the film is annealed in an oxidizing atmosphere, so that oxygen vacancies in the tantalum oxide film thus formed cannot completely be filled, thereby resulting in the capacitive insulating film having a poor film quality being formed when the capacitor is formed. Therefore, a leakage current flows trough the capacitor, to make it difficult for the capacitor to serve as an information storage capacitive element, resulting in deteriorated reliabilities of a relevant DRAM. In this case, in the tantalum oxide film manufacturing method disclosed in Japanese Patent Application Laid-open No. 2000-340559 as described above, if heat treatment for introducing the oxygen gas to supply a sufficient amount of oxygen to the oxygen vacancies in the formed tantalum oxide film is prolonged, a throughput is deteriorated inevitably. Furthermore, as duration of the heat treatment is prolonged, a surface of a polysilicon film or a like making up the lower electrode is oxidized, to form the silicon oxide film or a like having a small permittivity, thereby reducing a total capacitance value of the capacitor.
By such the conventional semiconductor device manufacturing method, it has been impossible to form the capacitive insulating film having good step coverage and a good film quality.